DELETE FROM oc_address;
DELETE FROM oc_category;
DELETE FROM oc_category_description;
DELETE FROM oc_category_to_store;
DELETE FROM oc_coupon;
DELETE FROM oc_customer;
DELETE FROM oc_download;
DELETE FROM oc_download_description;
DELETE FROM oc_manufacturer;
DELETE FROM oc_manufacturer_to_store;
DELETE FROM oc_product;
DELETE FROM oc_product_description;
DELETE FROM oc_product_discount;
DELETE FROM oc_product_featured;
DELETE FROM oc_product_image;
DELETE FROM oc_product_option;
DELETE FROM oc_product_option_description;
DELETE FROM oc_product_option_value;
DELETE FROM oc_product_option_value_description;
DELETE FROM oc_product_related;
DELETE FROM oc_product_special;
DELETE FROM oc_product_to_download;
DELETE FROM oc_product_to_store;
DELETE FROM oc_review;
DELETE FROM oc_store;
DELETE FROM oc_store_description;
DELETE FROM oc_product_tags;
DELETE FROM oc_order;

DELETE FROM oc_address;
DELETE FROM oc_category;
DELETE FROM oc_category_description;
DELETE FROM oc_category_to_store;
DELETE FROM oc_coupon;
DELETE FROM oc_customer;
DELETE FROM oc_download;
DELETE FROM oc_download_description;
DELETE FROM oc_manufacturer;
DELETE FROM oc_manufacturer_to_store;
DELETE FROM oc_product;
DELETE FROM oc_product_description;
DELETE FROM oc_product_discount;
DELETE FROM oc_product_featured;
DELETE FROM oc_product_image;
DELETE FROM oc_product_option;
DELETE FROM oc_product_option_description;
DELETE FROM oc_product_option_value;
DELETE FROM oc_product_option_value_description;
DELETE FROM oc_product_related;
DELETE FROM oc_product_special;
DELETE FROM oc_product_to_download;
DELETE FROM oc_product_to_store;
DELETE FROM oc_review;
DELETE FROM oc_store;
DELETE FROM oc_store_description;
DELETE FROM oc_product_tags;
DELETE FROM oc_order;
DELETE FROM oc_order_download;
DELETE FROM oc_order_history;
DELETE FROM oc_order_option;
DELETE FROM oc_order_product;
DELETE FROM oc_order_status;
DELETE FROM oc_order_total;
DELETE FROM oc_product_to_category;
DELETE FROM oc_coupon_description;
DELETE FROM oc_coupon_product;

This is another good idea on how to streamline the code for opencart. I know Q has been working his butt off to reduce large repetitive code to smaller chunks of code, hopefully this will make it into a future release of opencart